1. Field of the Invention
The present invention relates generally to the field of optical communication systems. More particularly, the present invention provides an N.times.M optical wavelength router for wavelength division multiplex (WDM) optical communications.
2. Statement of the Problem
Optical wavelength division multiplexing (WDM) has become the standard technique to fully utilize the high bandwidth provided by optical fibers. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on an optical fiber. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be simultaneously transmitted over a single optical fiber using WDM technology.
Despite the substantially higher fiber bandwidth utilization provided by WDM technology, a number of serious problems must be overcome if these systems are to become commercially viable (e.g., multiplexing, demultiplexing, and routing optical signals). The addition of the wavelength domain increases the complexity for network management because processing now involves both filtering and routing. Multiplexing is the process of combining multiple channels each defined by its own frequency spectrum into a single WDM signal. Demultiplexing is the opposite process in which a single WDM signal is decomposed into the individual channels. The individual channels are spatially separated and coupled to specific output ports. Routing differs from demultiplexing in that a router spatially separates the input optical channels into output ports and permutes these channels according to control signals to provide a desired coupling between an input channel and a specified output port.
One prior approach to wavelength routing has been to demultiplex the WDM signal into a number of component signals using a prism or diffraction grating. The component signals are each coupled to a plurality of 2.times.2 optical switches that are usually implemented using opto-mechanical switches. Optionally, a signal to be added to the WDM signal is coupled to one of the 2.times.2 switches. One output port of each 2.times.2 optical switch is coupled to a first multiplexer (the retained output multiplexer) that combines the retained signals and the added signal. A second signal from each 2.times.2 optical switch is coupled to a second multiplexer (the dropped signal multiplexer). By proper configuration of the optical switches, one signal can be coupled to the output port of the second multiplexer, while all the remaining signals pass through the output port of the first multiplexer. This structure is also known as an add-drop optical filter. The structure is complicated, relies on opto-mechanical switches, and interconnections tend to be difficult.
A second type of wavelength-selectable space switch is shown in U.S. Pat. No. 5,488,500 (Glance). The Glance filter provides the advantage of arbitrary channel arrangement but suffers significant optical coupling loss because of the two array waveguide grating (AWG) demultiplexers and two couplers used in the structure. Array waveguide gratings are one of the most popular technique in processing the WDM signals. This technology is based on planar waveguide silicon processing and has been widely adopted by the fiber optics industry. However, AWG is fundamentally passive, in that the output wavelength distribution is fixed by the WDM signals that are input at the input port. To perform the exchange of optical channels (i.e., routing), optical switches are needed and the required cascading of filters and switches makes this type of wavelength router cumbersome.
Another problem with prior approaches and with optical signal processing in general is high cross-talk between channels. Cross-talk occurs when optical energy from one channel causes a signal or noise to appear on another channel. Cross-talk must be minimized to provide reliable communication. Also, filters used in optical routing are often polarization dependent. The polarization dependency usually causes higher cross-talk as optical energy of particular polarization orientations may leak between channels or be difficult to spatially orient so that it can be properly launched into a selected output port. Similarly, optical filters provide imperfect pass band performance in that they provide too much attenuation, or signal compression at side lobes of the pass band is not high enough.
All of these shortcomings lead to imperfect or inefficient data communication using optical signals. What is needed is a routing structure that provides low cross-talk to eliminate unnecessary interference from other channels in a large network, flat pass band response in the optical spectrum of interest so that the wavelength router can tolerate small wavelength variations due to laser wavelength drift, polarization insensitivity, and moderate to fast switching speed for network routing. Also, a router with low insertion loss is desirable so that the router will minimally impact the network and limit the need for optical amplifiers.